FIG. 1 is a block diagram showing an example of the configuration of a conventional digital signal transmitting apparatus which performs m-ary modulation of modulating three or more bits of transmission data to one symbol. Conventional digital signal transmitting apparatus 10 shown in FIG. 1 includes coder 11 to which transmission information data is inputted from outside, rate matching section 12 which receives the output of coder 11, and m-ary modulation section 13 which receives the output of rate matching section 12.
Coder 11 performs error correction coding, such as convolution coding or turbo coding, on information data to be transmitted and gives the result to rate matching section 12.
Rate matching section 12 performs rate matching processing on transmission data after error correction coding, to make the number of bits of the transmission data equal to a predetermined number of bits per radio frame on a physical channel, and gives the result to m-ary modulation section 13. Specifically, in the rate matching processing, when the number of bits of transmission data after error correction coding is less than the predetermined number of bits per radio frame on a physical channel, repetition processing is performed to repetitively insert bits in the bit sequence of the transmission data after error correction coding in a given cycle. When the number of bits of transmission data after error correction coding is greater than the predetermined number of bits, puncturing processing is performed to extract bits from the bit sequence of the transmission data after error correction coding in a given cycle.
M-ary modulation section 13 converts the error-corrected coded data after the rate matching processing, to a m-ary modulation signal having a plurality of bits modulated into one symbol. When the modulation scheme is 16 QAM (Quadrature Amplitude Modulation), for example, four bits are modulated into one symbol as shown in FIG. 2. FIG. 2 provides signal constellation diagrams on an IQ plane in 16 QAM. In 16 QAM, bits are placed on the IQ plane per four bits, as shown in FIG. 2.
Referring to the intercede distance of the bits in FIG. 2, the first and second bits are greater than the third and fourth bits. That is, the first and second bits have higher error robustness and the third and fourth bits have lower error robustness. Bits of higher error robustness will be referred to as upper bits and bits of lower error robustness will be referred to as lower bits. This means that, on the receiving side, error correction becomes difficult due to the influence of the lower bits, and, consequently, the reception performance deteriorates.
The digital signal transmitting apparatus with the configuration shown in FIG. 1 which directly performs m-ary modulation on the data sequence after rate matching processing has a problem that, as shown in FIG. 2, error robustness (likelihood of error, intercede distance, etc.) varies according to bit positions, and, consequently, the reception performance deteriorates.
One way of coping with the problem that error robustness varies according to bit positions is to perform error correction coding only on lower bits having lower error robustness, to improve the transmission rate while preventing degradation in error rate characteristics (Patent Document 1, for example). Another way is to perform m-ary modulation by mapping coded data of higher coding rate to upper bits of higher error robustness and mapping coded data of lower coding rate to lower bits of lower error robustness, based on the level of error robustness according to bit positions (Patent Document 2, for example). Still another way is to switch upper bits and lower bits upon every retransmission and equalize error robustness according to bit positions after packet combining and improve error rate characteristics (Patent Document 3, for example).    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-186023    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-23691    Patent Document 3: Japanese Patent Application Laid-Open No. 2003-309535